Olefin polymerization catalysts comprising a metallocene and an aluminum alkyl component were first proposed in about 1956. Australian patent 220436 proposed for use as a polymerization catalyst a bis-(cyclopentadienyl) titanium, zirconium, or vanadium salt as reacted with a variety of halogenated or unhalogenated aluminum alkyl compounds. Although capable of catalyzing the polymerization of ethylene, such catalytic complexes, especially those made by reaction with a trialkyl aluminum, had an insufficient level of catalytic activity to be employed commercially for production of polyethylene or copolymers of ethylene.
Later it was found that certain metallocenes such as bis-(cyclopentadienyl) titanium, or zirconium dialkyls in combination with aluminum alkyl/water cocatalyst formed catalyst systems for the polymerization of ethylene. Such catalysts are discussed in German Patent Application 2,608,863 which discloses a polymerization catalyst for ethylene consisting of bis-(cyclopentadienyl) titanium dialkyl, trialkylaluminum and water. German Patent Application 2,608,933 discloses an ethylene polymerization catalyst consisting of a cyclopentadienyl zirconium salt, a trialkylaluminum cocatalyst and water. European Patent Application No. 0035242 discloses a process for preparing ethylene and atactic propylene polymers in the presence of a cyclopentadienyl transition metal salt and an alumoxane. Such catalysts have sufficient activity to be commercially useful and enable the control of polyolefin molecular weight by means other than hydrogen addition--such as by controlling the reaction temperature or by controlling the amount of cocatalyst alumoxane as such or as produced by the reaction of water with an aluminum alkyl.
To realize the benefits of such catalyst systems, one must use or produce the required alumoxane cocatalyst component. An alumoxane is produced by the reaction of an aluminum alkyl with water. The reaction of an aluminum alkyl with water is very rapid and highly exothermic. Because of the extreme violence of the reaction the alumoxane cocatalyst component has, heretofore, been separately prepared by one of two general methods. Alumoxanes may be prepared by adding an extremely finely divided water, such as in the form of a humid solvent, to a solution of aluminum alkyl in benzene or other aromatic hydrocarbons. The production of an alumoxane by such procedure requires use of explosion-proof equipment and very close control of the reaction conditions in order to reduce potential fire and explosion hazards. For this reason, it has been preferred to produce alumoxane by reacting an aluminum alkyl with a hydrated salt, such as hydrated copper sulfate. In such procedure a slurry of finely divided copper sulfate pentahydrate and toluene is formed and mantled under an inert gas. Aluminum alkyl is then slowly added to the slurry with stirring and the reaction mixture is maintained at room temperature for 24 to 48 hours during which a slow hydrolysis occurs by which alumoxane is produced. Although the production of alumoxane by a hydrated salt method significantly reduces the explosion and fire hazard inherent in the wet solvent production method, production of an alumoxane by reaction with a hydrated salt must be carried out as a process separate from that of producing the metallocene-alumoxane catalyst itself, is slow, and produces hazardous wastes that create disposal problems. Further, before the alumoxane can be used for the production of the an active catalyst complex the hydrated salt reagent must be separated from the alumoxane to prevent it from becoming entrained in the catalyst complex and thus contaminating any polymer produced therewith. In either of the above described methods, only trimethylaluminum (TMA) forms alumoxane with good catalytic activity, other alkylaluminums, such as triethylaluminum (TEAL), form alumoxane with very low catalytic activity.
U.S. Pat. No. 4,431,788 discloses a process for producing a starch filled polyolefin composition wherein a trialkyl aluminum is first reacted with starch particles of a mositure content below 6 weight percent. The starch particles are then treated with a (cyclopentadienyl)-chromium, titanium, vanadium or zirconium alkyl to form a metallocene-alumoxane catalyst complex on the surface of the starch particles. An olefin is then polymerized about the starch particles by solution or suspension polymerization procedures to form a free-flowing composition of polyolefin-coated starch particles. German Patent 3,240,382 likewise discloses a method for producing a filled polyolefin composition which utilizes the water content of an inorganic filler material to directly react with a trialkylaluminum and produce thereon an active metallocene-alumoxane catalyst complex. Polymer is produced by solution or gas phase procedures at the filler surface to uniformly coat the filler particles and provide a filled polymer composition.
German Patent 3,240,382 notes that the activity of a metallocene-alumoxane catalyst is greatly impaired or lost when prepared as a surface coating on an inorganic material. Although German Patent 3,240,382 suggests that an inorganic material containing absorbed or adsorbed water may be used as a filler material from which the alumoxane cocatalyst component may be prepared by direct reaction with a trialkylaluminum, the only water containing inorganic filler materials which are identified as capable of producing the alumoxane without adversely affecting the activity of the metallocene alumoxane catalyst complex are certain inorganic materials containing water of crystallization or bound water, such as gypsum or mica. German Patent 3,240,382 does not illustrate the production of a catalyst coated inorganic filler material wherein the inorganic material is one having absorbed or adsorbed water. Nor does German Patent 3,240,382 describe an inorganic filler material having absorbed or adsorbed water which has surface area or pore volume properties suitable for service as a catalyst support for a gas phase polymerization procedure.
My co-pending U.S. patent application Ser. No. 134,413 discloses a method by which the requisite alumoxane cocatalyst component for a supported metallocene gas phase polymerization catalyst may be safely and economically prepared by addition of an "undehydrated" silica gel to a trialkylaluminum solution. My co-pending application illustrates the production of highly active silica gel supported metallocene-alumoxane catalyst wherein trimethylaluminum is utilized to form the alumoxane. Although triethylaluminum is known to be ineffective for the separate production of an alumoxane cocatalyst to yield a highly active catalyst system with a metallocene, in accordance with the method disclosed in my co-pending application Ser. No. 134,413, triethylaluminum was found to be suitable for production of very active supported metallocene-alumoxane catalyst systems by directly reacting triethylaluminum with undehydrated silica.
Although the relatively low cost of triethylaluminum (TEAL) compared to trimethylaluminum (TMA) would seem to make TEAL preferred for use, such is still not the case when viewed in light of the difference in catalyst activity between a TMA produced catalyst system compared to a TEAL produced catalyst system. The supported catalyst produced exclusively with TMA always has an activity at least about 20% greater than a like catalyst system produced exclusively with TEAL. More typically, the activity of a TMA produced catalyst may be about 100% greater than a TEAL produced catalyst. Hence, from a commercial production standpoint, the catalyst activity difference still dictates the use of TMA for production of the alumoxane cocatalyst component dispite the cost savings that could be realized by use of TEAL as an alternative.
It would be desirable to devise an economical procedure whereby an active supported metallocene-alumoxane catalyst could be safely produced for use as a gas phase or liquid phase polymerization catalyst. To be economical the procedure should dispense with the requirement of producing the alumoxane component as a separate component apart from the procedure by which the catalyst itself is prepared. Further, it would be desirable to devise a procedure which utilizes TEAL to produce a substantial portion of the alumoxane cocatalyst required to provide a supported metallocene-alumoxane catalyst of an activity comparable to one wherein the requisite alumoxane cocatalyst component is formed exclusively from TMA.